This is simple utilitarianism, which is a simply idiotic philosophical system.
This is such obvious nonsense as to expose the stupidity of the advocate. Under a utilitarian system, if stealing from you makes you very happy, but losing your stuff to a thief makes you only somewhat annoyed, the theft is good because it increases the net amount of happiness in the world.
Exactly.
Under a libertarian system, the theft is evil because it violates the rights of the victim (no matter how little he might care, so long as he does not care so little as to actually consent to it).
I'm familiar with the argument. But if you notice in the original remarks there is no reference to "rights," just "happiness."
Several problems remain for the libertarian:
Where do rights come from?
Are they binding on everyone? If so, why?
By what authority do libertarians impose their idea of "rights" on society?